1. Field of the Invention
The invention relates to a carbon-based coating which contains in addition to carbon as the main component at least one first element selected from a group comprising the transition metals from groups 3 to 10 (new IUPAC nomenclature) of the periodic table of the elements, in particular from a group comprising or consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, wherein the carbon in the coating is mostly present in sp2-hybridized form, an object with a surface which has a coating, as well as a method for producing a carbon-based coating comprising in addition to carbon as the main component at least one first element selected from a group comprising groups 3 to 10 (new IUPAC nomenclature) of the periodic table of the elements, in particular from a group comprising or consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, on a substrate by means of sputtering by using at least one carbon target and at least one target of the first element, wherein the carbon in the coating is mostly in sp2-hybridized form.
2. Description of the Related Art
A method of this kind is already known from EP 1 036 208 B1. In this case a magnetron sputter ion-plating system is used with at least one carbon target, in which the ion current density applied to the substrate to be coated is set to be over 0.5 mA/cm2, which is high enough to apply a carbon layer in which the carbon-carbon-bond is mainly present in sp2-grahite form. In addition, at least one metal target made of titanium or chromium is used to apply a metal-containing coating layer, which has a layer thickness of between 50 and 200 nm. The carbon layer has a thickness of at most 1 μm. The substrate is rotated during the coating. In particular, three carbon targets and one metal target are used. In this way a sequence of alternating metal layers and carbon-containing layers is built up. At least two magnetrons are arranged so that a magnetic field is formed in between, wherein field lines run from one magnetron to the other said magnetron, and wherein the magnetrons and field lines which run directly from one said magnetron to the other form a barrier, which tends to prevent the “escape” of electrons from a plasma, which contains the working area in which the substrate is coated.
Said coating has very good mechanical properties, in particular tribological properties, with a specific wear rate in wet conditions of less than 10−16 m3/Nm, and is therefore used in the automobile industry, e.g. for gear wheels, camshafts, valves, piston rings or cylinder liners. Medical products, such as e.g. prostheses, are mentioned as a further field of application in EP 1 036 208 B1.
Although this coating has proven to be effective in practice it has been observed however that in higher temperature ranges its mechanical strength decreases, in particular its wearing resistance.